This relates in general to ultrasonic flowmeter systems and, more specifically, to the transducer assemblies employed in such systems, particularly for measurement of the flow of gas.
In ultrasonic flowmeter systems of a type disclosed in the prior art, the velocity of fluid flowing in a test pipe section is measured by transmitting a pulsed ultrasonic beam obliquely across the pipe section from a transmitting transducer interposed at one point in the pipe to a receiving transducer interposed at a second position in the pipe, spaced up or downstream from the first. When an ultrasonic wave is transmitted through the test fluid, the time required for such transmission is dependent on the velocity of the ultrasound in the fluid, the length of the path over which the beam is transmitted and the velocity at which the fluid is flowing. The velocity of fluid flow either adds to or substracts from the transmission time, depending on whether the beam is transmitted against or in the direction of the stream of flow. Thus, a feedback circuit produces a signal having a frequency or repetition which varies in accordance with the time required for the ultrasound waves to travel over a predetermined path in the fluid.
Such ultrasonic systems are well-known in the art, as disclosed, for example, in U.S. Pat. No. 2,669,121 to R. L. Garman et al, issued Feb. 16, 1954, and by others.
In accordance with prior art practice, the transducer used for transmitting and/or receiving the ultrasonic beam in such a system were so constructed that the maximum lobe of the emitted beam was directed in substantially a normal direction from the face of the transducer. Thus, in order to provide a beam capable of crossing the stream of fluid flowing in the pipe at an oblique angle, it was necessary to build a shoulder or supporting nipple onto the surface of the pipe. This obviously requires precise modifications in the pipes to which such a system is applied. Moreover, it creates a series of recesses in the inner pipe wall, associated with the obliquely positioned transducers which tend to cause interruptions or turbulence in the fluid flow being measured.
In addition, particularly in measurement of the flow rates of gases or low density liquids, problems arise in impedance matching the radiating member of the transducer assembly to the fluid in such a way as to avoid unwanted reflections and thereby energy loss.